Recovery of phthalic acids



Feb. 7, 1956 H. J. AROYAN ET AL 2,734,080

RECOVERY OF PHTHALIC ACIDS Filed July 12, 1954 5 Sheets-Sheet 1 0 2 o IN Z 9 2 LL.

o o w 10 v m N o D 92 .LV GHUQSVBW Hd lNVENTORS HARRY J. AROVAN JOHN B.W/LKES BY I'D- M ATT %EYS Feb. 7, 1956 H. J. AROYAN ET AL 2,734,080

RECOVERY OF PI-ITI-IALIC ACIDS Filed July 12, 1954 5 Sheets-Sheet 2/FIRST STAGE HYDROLYSIS INITIAL PH "'3 T0 3.5

fsecowo STAGE HYDROLYSIS L INITIAL PH 3 0.1

/THIRD STAGE HYDROLYSIS INITIAL PH '3 0 2O 4O 6O REACTION TIM E MINUTES119-3 INVENTORS HARRY J. AROI AN JOHN B. WILKES Feb. 7, 1956 H. J.AROYAN ETAL 2,734,080

RECOVERY OF PHTHALIC ACIDS Filed July 12, 1954 3 Sheets-Sheet 3 FIG. 3

. u) INVENTORS HARRY J. AROVAN JOHN B. WLKES BY 4% N rm 8- [K50 l I ATTOEYS United States Patent O RECOVERY OF PHTHALIC ACIDS Harry J. Aroyan,ElCerrito, and John B. Wilkes, Albany, Califl, assignors to CaliforniaResearch Corporation, San Francisco, Calif., a corporation of DelawareApplication July 12, 1954, Serial No. 442,694 19 Claims. (Cl. 260-625)This invention relates to a process for recovering phthalic acids frommixtures of the ammonium salts and amides of these acids.

This application is a continuation-in-part of our copending applicationSerial No. 257,434, filed November 20, 1951, now abandoned.

Copending application of William G. T oland, Ir., Serial No. 371,209,filed July 30, 1953, now United States Patent No. 2,722,549, describes aprocess for oxidizing alkyl aromatic hydrocarbons such as' the threexylene isomers, 1,3'-dimethyI-S-tertiary-butyI benzene, toluene, and thelike, by heating the alkyl aromatic hydrocarbons with ammonium sulfate,a sulfide'such as hydrogen sulfide or ammonium sulfide and water to atemperature from about 550 F. to the critical temperature of Water undera superatmospheric pressure sufficient to maintain a part of the waterin liquid phase. In a representative operation, 1 mol of xylene, 1.83mols of. ammonium sulfate, 0.18 mol of ammonium sulfide and 30 mols ofwater are heated to about 620 F. under a' pressure of 2800 p. s. i. g;for approximately 2 hours. 97% conversion of the xylene is achieved anda yield of phthalic acid derivatives amounting to 91% of theory isobtained. The reaction product produced in this oxidation process is amixture of ammoniumphthalates and phthalic acid amides. Three types ofcompound are present having the formulas:

(COONH4)2,. (CONH2)2 and NHZOCCOONH4 Thus, the reaction product containsdiammoni'um phthalate; diamide' of phthalic acid, and the half salt-halfamide of phthalic acid. In the reaction product from about 60 to 80% ofthe total potential acid groups is in the: form: of the ammonium saltand the remainder is in the form ofthc amide, i. e., 60 to 80% of theside chains of the reaction product is ammonium carboxylate --COONH4,while the remainder is amide CONH2. Throughout the specification andclaims the phrase mixtures of ammonium salts and amides of phthalicacids is'intended to describe mixtures of the" three compounds shownabove.

While the oxidation process above referred to is characterized by'highconversions of xylene and high yields ofphthalic acid values, therecovery of phthalic acid having the degree of purity required in mostcommercial uses fromtheicrude oxidation reaction product is difficult'.Aphthalic' acid product satisfactory for virtually" any commercial usemust have a nitrogencontent calculated as ammoniabel'ow about 0.1% byweight and for a great many uses, for example, in the production: ofalkyd resins, it is necessary that the nitrogen content calculated asammonia be below ;03% by weight. These low nitrogen contents are noteasily attained. They can be reached by saponifying the ammoniumsalt-amide product with caustic. as disclosed. in our copendin'gapplication Serial No. 257,434, filed November 20, 1951,10. produceasolu tion of sodium phthalates from which phthalic acids. areprecipitated by the addition of acid. This methodinvolves 2,734,080Patented Feb. 7 1056 considerably increases the ultimate cost of thephthalic acid product. It has been found also that the hydrolysis can beaccomplished in a strongly acid medium, i. e., 2 mols of sulfuric acidper mol of phthalic acid contained in the salt-amide mixture at hightemperature. This method, however, is attended by extraordinarily highcorrosion rates with the most corrosion resistant materials nowavailable requiring frequent replacement of expensive equipment withattendant reduction in the operating factor of the process.

It has now been found that mixtures of ammonium salts and amides ofphthalic acids can be hydrolyzed to produce phthalic acids of lownitrogen content by acidifying an aqueous solution of the mixture with amineral acid such as sulfuric, hydrochloric,- nitric and phosphoricacids, as disclosed in the aforesaid copending application to a pH inthe range from about 2.5 to about 3.5 (this pH range is attained byadding approximately 1 equivalent of sulfuric acid to the solution ofthe mixture for each equivalent of ammonium carboxylatecontained in themixture), heating the mixture so acidified to a temperature above 300 F.and desirably in the range from 400 F. to 500 F and preferably in therange from 430 F. to 500 F., under a superatmospheric pressuresutficient to maintain a substantial part of the water in liquid phasefor a period of about 10 to 60 minutes and then cooling the mixture to atemperature to below about 250 F., and filtering the cooled mixture torecover a filter cake consisting essentially of phthalic acids; It isdesirable to reacidifythe cooled mixture to pH 3 to-3.5 with a strongmineral acid prior to filtration. A single-stage treatment in thismanner serves to reduce the nitrogen content of the phthalic acidproduct, determined as ammonia, from an initial value ordinarily in therange from 6 to 10% to a value in the range from about 0.6% to 1%.Further reduction in the nitrogen content maybe obtained by slurryingthe phthalic acid filter cake with water to produce a slurry having asolids content from about 2% to about byweight, preferably from 10% to25% by weight, and heating the slurry to a temperature above 300 F. andpreferably in the range from about 430 F. to 500 F. For certain uses ofthe phthalic acid requiring extremely high purity, a third-hydrolysisstage may be conducted in a similar manner.

Pursuantto another embodiment of the invention, the phthalic acid filtercake obtained in the first hydrolysis stage is slurried with a quantityof an aqueous solution of a bisulfate selected from the group consistingof ammonium bisulfate and alkali metal bisulfates suflicient to producea slurry having a' solids content in the range from about 2% to about50% by weight, preferablyfrom about 10% to about 25 by weight, andcharacterized v by a pH in the range from about 1.0to about 1.8. Thissubstantial consumption of caustic and'sulfuric acid which 7 slurry isthen' heated to a temperature above 300 F. and preferably in the rangefrom about 350 F. to 525 F., and preferably from 400 F. to 500 F.',under a superatmospheric pressure sufiicient to maintain the water inliquid phase, cooled, and filtered to recover a filter cake consistingessentially of phthalic acid. The desired pH range, i. e., 1.2 to 1.8,in thisemb'o'ditnent of the invention, is obtained by controlling"thebisulfate content of the slurry so that it contains approximately 1mol of bisulfate per mol of nitrogen, determined as ammonia, which iscontained in the filter cake: obtained in the first stage of hydrolysisandby the addition of some excess neutral sulfates which help to butterthe mixture.

Figure 1 of the appended. drawings is a graphical illustration-ofthechange in. pH which occurs as sulfuric acid is addedto asolution of amixture of ammonium salts and amidesof phthalic acids. Figure 2 of. theappended drawings is a graphical illustration of the reduction in thenitrogen content of a mixture of ammonium salts and ayrsapso amides ofphthalic acids which is obtained in three hydrolysis stages carried outin the presence of any mineral acid. Figure 3 of the appended drawingsis a diagrammatic illustration of arrangement of apparatus and processflowsuitable for the practice of the invention.

A typical ammonium phthalate-phthalic acid amide mixture producedpursuant to the process described in the above-mentioned Tolandapplication is titrated with 19 normal sulfuric acid. The titrationcurve is shown in Figure 1. The phthalic acid salt-amide solution had acalculated phthalic acid content of 15.5 g. per 100 g. of solution, thephthalic acid values in the solution being a mixture of isophthalic acidand terephthalic acid and containing about 95% isophthalic acid. Fromthe titration curve it will be observed that a rapid drop in pH occursas the total acid added to the ammonium saltamide solution rose from 20to 24 cc. This drop in pH occurs when approximately one equivalent ofsulfuric acid has been added for each equivalent of ammonium carboxylatecontained in the ammonium salt-amide mixture. After the addition of thisquantity of sulfuric acid, substantially all of the ammonium carboxylategroups contained in the solution have undergone a replacement of theammonium radical by hydrogen and exist as carboxyl groups. When thisquantity of acid has been added, i. e., when the pH of the ammoniumsalt-amide solution has been adjusted to a value in the range from about2.5 to 3.5, the solution is in proper condition for heating to atemperature above 300 F., and preferably in the range from 400 to 500F., to cause hydrolysis of the amide groups. When the calculatedphthalic acid content of the salt-amide solution is in the range fromabout 8 to 25% by weight, it is desirable to add the sulfuric acid at anelevated temperature, for example, a temperature in the range from about150 F. to 250 F., to avoid the production of a viscous slurry asphthalic acids and amides are precipitated as a result of the acidaddition. While the actual acid addition is desirably made at thesehigher temperatures, the pH values employed in controlling the acidaddition are determined at about 70 F. Throughout the specification andclaims it should be understood that the pH values recited are pH valuesdetermined at about 70 F. and that the actual pH existing at the highertemperatures may differ considerably from the value measured at 70 F.

Figure 2 of the appended drawings shows the reduction in nitrogencontent of a mixture of ammonium salts and amides of phthalic acids inthree stages of hydrolysis. In each of the three hydrolysis stages theacidified salt-amide mixtures were heated to 465 F. under a pressure of500 p. s. i. g. for 60 minutes and samples were taken for analysis asthehydrolysis progressed. In all stages the initial pH of the mixtureswas in the range from 3 to 4. During the 60 minute hydrolysis period thepH rose to a level in the range from 4.7 to 5.2 as amide groups werehydrolyzed to ammonium carboxylate groups and the ammonia liberated fromthe amide groups utilized hydrogen ions to form ammonium radicals. Atthe end of the first stage the reaction mixture was cooled to 180 F.,acidified to pH below 4.0, and filtered. The nitrogen content of thefirst-stage filter cake expressed as ammonia was 0.74%. The firstastagefilter cake was reslurried with water and this slurry was then heated tocause further hydrolysis. The second stage filter cake, obtained byfiltering after adjusting the pH of the second-stage product to a valueof 3 to 4, had a nitrogen content of 0.05 expressed as ammonia. Thethird hydrolysis stage was conducted by reslurrying the second stagehydrolysis filter cake in water, and heating to cause furtherhydrolysis. The final phthalic acid producthad a nitrogen content of0.013% expressed as ammonia. For many uses of the isophthalic acid'andterephthalic acid hydrolysis product, nitrogen contents below about0.06% by weight expressed as ammonia are satisfactory, while a phthalicacid product having a nitrogen content below 0.03% expressed as ammoniaappears to be satisfactory in the manufacture of alkyd resins wherepurity requirements are most severe.

Corrosion rates of stainless steels were determined under the conditionsof the first-stage hydrolysis illustrated in Figure 2. The initialcorrosion rate during the first hydrolysis stage when the pH is about 3was 26 mils per year for type 304 stainless steel and 29 mils per yearfor type 316 stainless steel. The corrosion rates for both of thesestainless steels were below 20 .mils per year after the first fewminutes of the hydrolysis treatment when the pH of the hydrolysisreaction mixture had risen to 4.0.

Isophthalic acid, terephthalic acid, and mixtures of these acids, havingnitrogen contents below 0.03%, determined as ammonia, can readily beobtained employing two hydrolysis stages. The first hydrolysis stage isconducted as illustrated in Figure 2 of the drawings by adjusting the pHof an aqueous solution of a mixture of ammonium salts and amides ofisoand terephthalic acids to a value in the range from 2.5 to 3.5 andheating the acidified mixture to 450 to 500 F. under a pressure of about500 p. s. i. g. for 10 to 60 minutes. The hydrolysis reaction mixture isthen cooled to a temperature below about 250 F. and sulfuric acid isadded to adjust the pH of the hydrolysis reaction product to a value inthe range from 2.5 to 3.5 (pH determined at F.). The cooled reactionproduct is then filtered and the filter cake is slurried with an aqueousbisulfate solution and heated to 350 F. to 500 F. under superatmosphericpressure to complete the hydrolysis. The second hydrolysis stageemploying aqueous bisulfate solution is illustrated by the followingexamples.

Example The filter cake obtained in the first-stage hydrolysisillustrated in Figure 2, the first-stage hydrolysis product having beenacidified to a pH of 3 prior to filtration, was slurried with watercontaining 13.2 g. of sodium sulfate per liter and 4.65 g. of sulfuricacid per liter. The calculated isophthalic acid content of the slurrywas g. per liter. This slurry had a pH of 1.45 measured at 70 F. Theslurry was heated to 465 F. under a pressure of about 500 p. s.i. g.Samples were withdrawn and analyzed for nitrogen as the hydrolysisprogressed. At the end of 30 minutes the nitrogen content of ,theisophthalic acid product expressed as ammonia had been reduced to 0.036%by weight. At the end of 60 minutes the nitrogen content similarlyexpressed was reduced to 0.022% by weight. At the end of the 60 minuteperiod, the pH of the hydrolysis reaction product measured at 70 F. hadrisen to 2.7.

Example 2 Example 1 was repeated, slurrying the first-stage isophthalicacid filter cake with water containing 26 g. per liter of sodium sulfateand 9.5 g. per liter of sulfuric acid. The isophthalic acid content ofthe slurry was again 170 g. per liter of water. The pH of the initialslurry at 70 F. was 1.27. At then end of 30 minutes heating at 465 F.,the nitrogen content of the isophthalic acid expressed as ammonia was0.03% by weight, and at the end of 60 minutes the nitrogencontent,-similarly expressed, had been reduced to 0.018% by Weight. -Atthe end of 60 minutes the pH of the hydrolysis reaction product hadrisen to 1.67. The initial corrosion rate for type 304 stainless steelunder the conditions of this example was 31 mils per year, but this ratedecreases as the acidity decreases during hydrolysis. It appears thatthis low corrosion rate must be attributed to the fact that sodiumbisulfate ion has a much lower dissociation constant at high temperaturethan at low temperature. For example, the constant for the ionization ofbisulfate ion to hydrogen ion and sulfate ion is 2 10- at roomtemperature, while at 400 F. the

ionization constant is estimated to be approximately lXl- This unusualproperty of bisulfate ion enables it. to provide a reservoir of hydrogenion for carrying out thehydrolysis of the amide groups withoutincreasing the acidity of the hydrolysis reaction mixture to high levelsat which corrosion rates become prohibitively high.

Figure 3 of the appended drawings illustrates an arrangement ofapparatus and process flow suitable for the practice of a two-stagehydrolysis of mixtures of ammonium salts and amides of isophthalic andterephthalic acids. A crude reaction product produced by oxidizing 95%meta-xylene pursuant to Toland application Serial No. 371,209, mentionedabove, is passed through line 1 into vessel 2. 96% sulfuric acid ispassed into line 1 through line 3 where it mixes with the aqueousmixture of. ammonium salts and amides of isophthalic acid. The quantityof sulfuric acid introduced through line 3 is adjusted so that themixture produced in vessel 2 has a pH. in the range from 2.5 to 3.5. Atypical feed stock introduced through line 1 will have a calculatedisophthalic acid content approximately of the total feed. Minor amountsof orthophthalic acid, benzoic acid and toluic acid will be contained inthe feed as a result of the presence of ortho-xylene and ethylbenzene inthe meta-xylene subjected to oxidation and to partial oxidation of avery small proportion of the metaxylene. Approximately one-half pound ofsulfuric acid per calculated pound of phthalic acids contained in thefeed is introduced into vessel 2. Vessel 2 is operated at about 200 to250 F. and the contents of the vessel are strongly agitated to insuregood mixing of the sulfuric acid and the feed. The acidified feed.having a pH of 2.5 to 3.5 is withdrawn from vessel 2 via line 4, passedthrough heat exchanger 5 into hydrolyzer 6. In heat exchanger 5 theacidified feed is heated to a temperature of about 400 to 450 F. Livesteam is introduced into hydrolysis vessel 6 through line 7 to agitatethe hydrolysis mixture and to raise its temperature to a level intherange from 450 to 500 F. The hydrolysis reactionv mixture flows fromhydrolyzcr 6 through line 8 into a second hydrolyzer 9. Sulfuric acid orammonium bisulfate solutions may be introduced into line- 8 through line10 in amount sufficient to drop the pH of the liquid in line 8 fromapproximately 5 to below about 3.5. The residence time of the reactionmixture in the hydrolysis vessels is ordinarily from 10 minutes to 1hour, being usually from about to minutes. Cold filtrate recovered asdescribed hereinafter is introduced into the lower portion of hydrolysisvessel 9 through line 11, the temperature and amount of this streambeing i adjusted to cool the hydrolysis reaction mixture in the lowerportion of hydrolysis vessel 9 to about 250 F. or below. The hydrolysisvessels are operated at 500 to 700 p. s. i. g. to maintain the watercomponent of the hydrolysis reaction mixture in liquid phase. The cooledhydrolysis reaction mixture is withdrawn from hydrolysis vessel 9through line 12 and is passed into cooler 13. Sulfuric acid isintroduced into cooler 13 through line 14 in amount sufficient to bringthe pH of the hydrolysis reaction product to a value in the range from2.5 to 3.5. A portion of the Water contained in the hydrolysis reactionproduct is evaporated in cooler 13 and the vapors are withdrawn throughline 15. The cooled slurry produced in cooler 13 is withdrawn throughline 16 and passed to filter 17 which may be either a conventional typefilter or a centrifugal filter. The filtrate produced at filter 17is-passed through line 18 into filtrate storage tank 19. A part of thefiltrate is withdrawn from tank 19 through line 20 and is passed intocooler 21. A portion of the filtrate is withdrawn from the hydrolysissystem: through line 22. Benzoic and toluic acids are extracted fromthis stream (line 22) and the. residual aqueous solution, which is.principally aqueous ammonium sulfate. can be returned to the oxidationunit in which meta-xylene is subjected to oxidation. where it. is.employed as the oxidizing agent. The liquid in cooler 21 is cooledordinarily to a temperature in the range from 70 to F. and the coldfiltrate is withdrawn from cooler 21 and passed into the lower portionof hydrolysis vessel 9 through line 11 as described above. The filtercake produced at filter 17 is passed through lines 23 and 24 into repulpvessel 25 where it is slurried with water containing about 2.5% byweightof sodium sulfate and about 0.8 to 1.0% by weight of sulfuric acid. Thewater containing sodium sulfate and sulfuric acid, i. e., sodiumbisulfate with excess sodium sulfate, is passed through line 27 intoline 24 and repulp vessel 25. The slurry produced in vessel 25 iswithdrawnfrom the vessel through line 28 and is passed-into manifoldline 29. Vessels 30, 31 and 32 are batch hydrolysis vessels, havingconnecting lines so arranged that simultaneously one vessel is beingfilled from manifold line 29 and heated to hydrolysis temperature, thesecond. vessel is being held at 450 to 525 F. under a pressure fromabout 500 to 700 p. s. i. g. to accomplish the hydrolysis, and the thirdvessel containing hydrolyzed slurry is being emptied. Live steam ispassed into manifold line 33 from which it can be passed into the vesselwhich is being filled and heated. Steam from the hydrolysis vessels iswithdrawn through manifold line 34 and line 35. During the cycleillustrated in the drawing, vessel 30 is being filled with slurry frommanifold line 29 and heated with steam from steam manifold line 33. Thefilling and heating are continued until the vessel is filled to workingcapacity and the stock has been heated to a temperature from 350 to 500F. In the illustrated cycle, vessel 31 is being held under pressure at atemperature in the range from 350 to 500 F. to hydrolize the amidecontained in the slurry. In the illustrated cycle, vessel 32 is cooledfrom hydrolysis temperature to about 250 F. by evaporation of waterwhich is removed through manifold line 34 and line 35. The cooledhydrolysis product produced in vessel 32 is withdrawn through line 36and passed. into slurry tank 37. Slurry is withdrawn from tank 37through line 38 and passed to filter 39. Filter cake is withdrawnthrough line 40 and filtrate through line 41.

In the cycle succeeding the illustrated cycle, tank 32 will be fillingand heating, the contents of tank 30 will be held at hydrolysistemperature and pressure, and the contents of tank 31 will be cooled andwithdrawn.

Isophthalic acid and terephthalic acid produced from mixtures of theirammonium salts and amides, in the manner described with reference to-Figure 3 of the drawings have nitrogen contents well below 0.03%detcrmined as ammonia. When these products are substituted for phthalicanhydride in conventional alkyd resin cooks, the resin is free of tarand sulfide precipitates and shows Gardner colors in the range from 4 to6. Similarly, orthophthalic acid, tertiary-butyl isophthalic acid andtertiary-butyl ortho-phthalicv acid having extremely low nitrogencontents can be produced from mixtures of their ammonium salts andamidesv pursuant to the invention. Mixtures of the ammonium salts andamides of tertiary-butyl isophthalic acid and tertiary-butylorthophthalic acid are produced. when LS-dimethyi-S-tertiarybutyl'benzene and l,2-dimethyl-4-tertiary butyl benzene, respectively, aresubjected to oxidation in the manner described in the Toland applicationabove referred to. While the process. of. the invention is especiallywell adapted to recovering pure phthalic acids from mixtures of theammonium salts and amides of phthalic acids, which are characterized bylimited water solubility, it may also be employed to recover otheraromatic acids such as benzoic acid and toluic acid from mixtures oftheir. ammonium salts and amides. I The term phthalic acids as employedin the appended claims isintended to. comprehendv not only the. threeisomeric phthalic acids themselves, but also thetertiary-butylesubstituted isophthalic and ortho-phthalic acids whichappear as ars igoso 7 mixtures of their ammonium salts and amides Whentertiary-butyl xylenes are subjected to the above-described oxidationprocess.

The second hydrolysis stage employing an aqueous mixture of sodiumbisulfate and sodium sulfate may be carried out in a continuous mannerparalleling the firststage operation, if desired. However, the batchhydrolysis illustrated in the drawing has the advantage of providingtime for phthalic acid crystal growth during the period when a batchreaction vessel is being ed and emptied. By cooling the hot hydrolysisproduct at a rate not exceeding about 15 Fahrenheit degrees per minute,the major proportion of the solid acid is larger than 200 mesh size.Crystals of this size are readily filtered from the hydrolysis reactionproduct and are adaptable to use in alkyd resin manufacture as such. incontinuous operation, smaller crystals are obtained which must besubjected to a flaking treatment in order to increase the particle sizeof the solid acid sufiiciently to make it readily usable in conventionalalkyd cooking.

We claim:

1. A process for recovering phthalic acids from mixtures of ammoniumsalts and amides of these acids which comprises acidifying an aqueoussolution of said mixture with sulfuric acid to a pH in the range fromabout 2.5 to about 3.5, heating the acidified mixture to a temperaturein the range from 300 to about 500 F. under superatmospheric pressuresufficient to maintain a substantial part of the water in liquid phase,cooling the mixture to a temperature below about 250 F., adding aquantity of sulfuric acid to the cooled mixture sufficient to lower thepH of the mixture to a value in the range from about 2.5 to about 3.5,filtering the resultant mixture to separate a filter cake consistingessentially of phthalic acid, mixing the filter cake with sufficientwater to form a slurry having a solids content from about 2% to about50% by weight, heating the slurry to a temperature in the range from 430to 500 F. under a superatmospheric pressure sufiicient to maintain asubstantial part of the water in liquid phase, cooling the heated slurryto a temperature below about 250 F. and filtering the cooled slurry torecover a filter cake consisting essentially of phthalic acids.

2. A process for recovering phthalic acids from mixtures of ammoniumsalts and amides of these acids which comprises adding to an aqueoussolution of said mixture approximately one equivalent of sulfuric acidper equivalent of ammonium carboxylate contained in said mixture,heating the resultant mixture to a temperature in the range from 300 to500 F. under a superatmospheric pressure suflicient to maintain asubstantial part of the water in liquid phase, cooling the mixture andadding to the cooled mixture approximately one equivalent'of sulfuricacid for each equivalent of ammonium carboxylate contained in saidcooled mixture, filtering the resultant mixture to separate a filtercake consisting essentially of phthalic acid, slurrying the filter cakewith a quantity of water suflicient to produce a slurry having a solidscontent in the range from about 2% to about 50% by weight, heating theslurry to a temperature in the range from about 430 F. to about 500 F.,cooling the hot mixture and filtering the cooled mixture to separate afilter cake consisting essentially of phthalic acids.

3. A process for recovering phthalic acids from mix tures of ammoniumsalts and amides of these acids which comprises acidifying an aqueoussolution of said mixture with sulfuric acid to a pH in the range fromabout 2.5 to about 3.5, heating the acidified mixtureto a temperature inthe range from 300 F. to about 500 F. under a superatmospheric pressuresufiicient to maintain a substantial part of the water in liquid phase,cooling the mixture and filteringthe cooled mixture to recover a filtercalie consisting essentially of phthalic acid, slurrying the filter cakewith a quantity of an aqueous solution of a bisulfate selected'frorn thegroup consisting of ammonium bisulfate and alkali metal bisulfatessuificient to produce a slurry having a solids content in the range fromabout 2% to about 50% by weight, and characterized'by a pH in the rangefrom about 1.2 to about 1.8, heating the slurry to a temperature in therange from about 350 F. to 500 F., cooling the hot mixture and filteringit to recover a filter cake consisting essentially of phthalic acids.

4. A process for recovering phthalic acids from mixtures of ammoniumsalts and amides of these acids which comprises acidifying an aqueoussolution of said mixture with sunurr'c acid to a pH in the range fromabout 2.5 to about 3.5, heating the acidified mixture to a temperaturein the range from 300 F. to about 500 F. under a superetmosphericpressure sufiicient to maintain a substantial part of the water inliquid phase, cooling the mixture, acidifying the cooled mixture withsulfuric acid to a pH in the range from about 2.5 to about 3.5, andfiltering the acidified mixture to recover a filter cake consistingessentially of phthalic acid, slurrying the filter cake with a quantityof an aqueous solution of a bisulfate selected from the group consistingof ammonium bisulfate and alkali metal bisulfates sufiicient to producea slurry having a solids content in the range from about 2% to about 50%by weight, and characterized by a pH in the range from about 1.2 toabout 1.8, heating the slurry to a temperature in the range from about350 F. to 500 F, cooling the hot mixture and filtering it to recover afilter cake consisting essentially of phthalic acids.

5. A process for recovering phthalic acids from mixtures of ammoniumsalts and amides of these acids which comprises adding to an aqueoussolution of said mixture approximately one equivalent of sulfuric acidper equivalent of ammonium carboxylate contained in said mixture,heating the resultant mixture to a temperature in the range from about400 F. to 500 F. under a superatmospheric pressure sufiicient tomaintain a substantial part of the water in liquid phase, cooling themixture, adding to the cooled mixture approximately one equivalent ofsulfuric acid per equivalent of ammonium carboxylate contained in saidmixture, and filtering the resultant mixture to recover a filter cakeconsisting essentially of phthalic acid, slurrying the filter cake withan aqueous bisulfate soiutio-n, said bisulfate solution being a solutionof a bisulfate selected from the group consisting of ammonium bisulfateand alkali metal bisulfates, the volume of said bisulfate solution beingsuch that the resultant slurry has a solids content in the range fromabout 2% to about 50% by weight and the bisulfate content of thebisulfate solution employed in forming the slurry being approximatelyone mol of bisulfate per mol of nitrogen deter: mined as ammoniacontained in said filter cake, heating said slurry to a temperature inthe range from about 350 F. to about 500 F., cooling the heated slurryand filtering it to recover a filter cake consisting essentially ofphthalic acids.

6. A process for recovering highly purified aromatic polycarboxylicacids characterized by low solubility in water, from the amides of theseacids, which comprises digesting the amide with an aoueous solution ofan inorganic hydrolyst to liberate the free acid, filtering the reactionproduct to recover a filter cake comprising the free acid, digesting thefilter cake with water at a temperature above 300 F. and under asuperatrnospheric pres sure sul'licient to maintain water in liquidphase and filtering the digestion product to recover the purified acid.

7. A process for recovering highly purified aromatic polycarbox-ylicacids characterized by low solubility in water, from the amides oftheseacids, which comprises hydrolyzing the amide to separate the freeacid, filtering the reaction product to recover a filter cake comprisingthe free acid, digesting the filter cake with liquid water at atemperature in the range 300 F. to 500 F. and filtering the digestedproduct to recover the purified acid.

'8. A process for recovering highly purified isophthalic acid andterephthalic acid from the amides of these-acids,-

9 which comprises digesting the amides with an aqueous solution of astrong mineral acid to liberate the free organic acid, filtering thereaction product to recover the filter cake comprising the free acid,digesting the filter cake with liquid water at a temperature above 300F. and under a superatmospheric pressure suificient to maintain thewater in liquid phase, the quantity of water employed beingsubstantially less than the amount required to dissolve the acid, andfiltering the digestion product to recover the purified acid.

9. A process for recovering highly purified isophthalic acid andterephthalic acid from the reaction product obtained by oxidizingmeta-xylene and para-xylene with ammonium sulfate, water and awater-soluble sulfide, which comprises digesting the reaction productwith an aqueous solution of a strong base to hydrolyze ammonium saltsand amides and form phthalic acid salts, acidifying the resultingmixture to precipitate free phthalic acids, filtering the acidifiedproduct to recover a filter cake comprising free phthalic acids,slurrying the filter cake with suflicient liquid water to form a slurrycontaining from 10 to 40 per cent by weight of solid phthalic acids,heating the slurry to a temperature from about 300 F. to 525 F. under asuperatmospheric pressure sufiicient to maintain the water in liquidphase and filtering the slurry to recover purified phthalic acids.

10. A process for recovering highly purified isophthalic andterephthalic acids from the reaction product obtained by oxidizingmeta-xylene and para-xylene .with ammonium sulfate, water and awater-soluble sulfide, which comprises digesting the reaction productwith aqueous sulfuric acid to liberate free phthalic acids, filteringthe resultant mixture to recover a filter cake comprising free phthalicacids, slurrying the filter cake with water to form a slurry containingfrom about 10 to 40 per cent by weight of solid phthalic acids,digesting the slurry at a temperature from about 300 F. to 525 F. undera superatmospheric pressure suflicient to maintain water in liquid phaseand filtering the digestion product to recover purified phthalic acids.

11. A process for recovering highly purified isophthalic acid andterephthalic acid from the amides of these acids which comprisesdigesting the amides with an aqueous solution of a strong mineral acidto hydrolyze the amide, filtering the reaction product to recover a freeacid product, digesting the filter cake with suflicient liquid water toform a slurry containing from about 5 to about by weight solids at atemperature above 300 F. and under a superatmospheric pressuresuificient to maintain the water in liquid phase, the quantity of wateremployed being substantially less than the amount required to dissolveall of the acid, and filtering the digestion product to recover purifiedphthalic acid.

12. A process for recovering highly purified aromatic polycarboxylicacids characterized by low solubility in water, from the amides of theseacids, which comprises hydrolyzing the amides to separate the freeacids, filtering the reaction product to recover a filter cakecomprising the free acids, digesting the filter cake with a liquidaqueous medium selected from the group consisting of water,

a solution of mineral acid, a solution of ammonium bisulfate and asolution of an alkali metal bisnlfate at a temperature in the range ofabout 300 F. to about 500 F., and under sufiicient superatmosphericpressure to maintain water in the liquid phase, and filtering thedigested product to recover the purified polycarboxylic acids.

13. The process of claim 12 wherein the amides are bydrolyzed withsulfuric acid.

14. The process of claim 12 wherein the amides are hydrolyzed withphosphoric acid.

15. The process of claim 12 wherein the filter cake is digested withaqueous sulfuric acid.

16. The process of claim 12 wherein the filter cake is digested with anaqueous solution of bisulfate.

17. The process of claim 12 wherein the amides are mixed with a mineralacid to form a mixture having a pH below about 4.0 and the amides arehydrolyzed by digesting the mixture at a temperature above about 300 F.and under a superatmospheric pressure suflicient to maintain water inliquid phase.

18. The process of claim 17 wherein the acid is sulfuric acid.

19. The process of claim 12 wherein the amides comprise isoandtere-phthalic acid amides.

No references cited.

1. A PROCESS FOR THE RECOVERING PHTHALIC ACIDS FROM MIXTURES OF AMMONIUMSALTS AND AMIDES OF THESE ACIDS WHICH COMPRISES ACIDIFYING AN AQUEOUSSOLUTION OF SAID MIXTURE WITH SULFURIC ACID TO A PH IN THE RANGE FROMABOUT 2.5 TO ABOUT 3.5, HEATING THE ACIDIFIED MIXTURE TO A TEMPERATUREIN THE RANGE FROM 300 TO ABOUT 500* F. UNDER SUPERATMOSPHERIC PRESSURESUFFICIENT TO MAINTAIN A SUBSTANTIAL PART OF THE WATER IN LIQUID PHASE,COOLING THE MIXTURE TO A TEMPERATURE BELOW ABOUT 250* F., ADDING AQUANTITY OF SULFURIC ACID TO THE COOLED MIXTURE SUFFICIENT TO LOWER THEPH OF THE MIXTURE TO A VALUE IN THE RANGE FROM ABOUT 2.5 TO 3.5,FILTERING THE RESULTANT MIXTURE TO SEPARATE A FILTER CAKE CONSISTINGESSENTIALLY OF PHTHALIC ACID, MIXING THE FILTER CAKE WITH SUFFICIENTWATER TO FORM A SLURRY HAVING A SOLIDS CONTENT FROM ABOUT 2% TO ABOUT50% BY WEIGHT, HEATING THE SLURRY TO A TEMPERATURE IN THE RANGE FROM 430TO 500* F. UNDER A SUPERATMOSPHERIC PRESSURE SUFFICIENT TO MAINTAIN ASUBSTANTIAL PART OF THE WATER IN LIQUID PHASE, COOLING THE HEATED SLURRYTO A TEMPERATURE BELOW ABOUT 250* F. AND FILTERING THE COOLED SLURRY TORECOVER A FILTER CAKE CONSISTING ESSENTIALLY OF PHTHALIC ACIDS.